1. Field of the Invention
The present invention generally relates to a magnetic encoder used in, for example, a rotation detecting device for detecting the rotational speed of bearing elements rotatable relative to each other and a wheel bearing assembly utilizing such magnetic encoder. In particular, the present invention relates to the magnetic encoder that forms one of component parts of a bearing sealing device that can be mounted in the rotation detecting device employed in an anti-skid control system for a motor vehicle for detecting the rotational speed of front and rear vehicle wheels.
2. Description of the Prior Art
The rotation detecting device for use in association with an anti-skid control system generally used in, for example, motor vehicles has hitherto been available in various types. Of them, the rotation detecting device has been known which includes a toothed rotor and a rotation detecting sensor that are separated from each other by means of a sealing device used to seal a bearing assembly. This known rotation detecting device is separate from and independent of the sealing device used in the bearing assembly.
This known rotation detecting device is of a structure wherein the rotational speed (the number of revolutions) of the toothed rotor mounted on a rotatable shaft is detected by the rotation detecting sensor mounted on a knuckle, and the bearing assembly used is protected from any possible ingress of water and/or any other foreign matter by means of the sealing device independently provided laterally of the rotation detecting device.
A different type is disclosed in, for example, the Japanese Patent No. 2816783, in which for reducing the space for mounting of the rotation detecting device to thereby drastically increase the sensing performance of the rotation detecting device, the rotation detecting device for detecting the rotational speed of a wheel is incorporated in a bearing seal unit. This bearing seal unit is of a structure in which an elastic member mixed with a powdery magnetic material is bonded radially by vulcanization to a slinger used therein so as to extend circumferentially, which elastic member has a plurality of opposite magnetic poles alternating with each other in a direction circumferentially thereof.
The Japanese Laid-open Patent Publication No. 6-281018 (U.S. Pat. No. 5,431,413) discloses the structure in which for reducing the dimension in an axial direction to increase the sealability between a rotatable member and a stationary member and also to facilitate mounting, a space between the rotatable member and the stationary member is sealed with a rotary disc mounted on the rotatable member while the rotary disc is provided with a coder magnetized to a plurality of opposite magnetic poles, to thereby complete a coder incorporated sealing structure. The coder used therein is made of an elastomer added with magnetic particles and has its side surface rendered to be a sealing means that is in flush with a side surface of the stationary member.
The coder made of a plastic material (plastomer) containing a powdery magnetic material or magnetic particles is shaped using a mold assembly adapted to the shape of a final product, that is, molded to the shape defined by the molding cavity within the mold assembly such as performed with the conventional injection molding or the compressive molding, or molded to the shape of a final product by means of an extrusion molding technique using a T-shaped die, or is first prepared by a sheet molding technique such as a calendaring technique in the form of a sheet that is then shaped by means of a blanking technique to the shape of a final product, which final product may be subsequently fixedly bonded to a metallic substrate with the use of a bonding agent. In such case, while a metallic substrate is incorporated in the mold assembly such as an insert molding, molten resin may be subsequently poured into the mold assembly so that a bonding step can be performed simultaneously.
However, of the various prior arts discussed above, the bearing seal device disclosed in any one of the Japanese Patent No. 2816783 and the Japanese Laid-open Patent Publication No. 6-281018 (U.S. Pat. No. 5,431,413) requires the use of an elastomer or an elastic material component that serves as a binder for retaining the powdery magnetic material or the magnetic particles. This is particularly true where the elastic material mixed with the powdery material is bonded radially by vulcanization to the slinger so as to extend circumferentially thereof or the coder defining the coder equipped sealing structure equipped with the coder magnetized to the opposite magnetic poles is rendered to be an elastomer added with the magnetic particles. However, where the elastomer or the elastic material component is used as a binder, it is always necessary to use a dispersing step in which prior to the shaping to the shape of the coder the powdery magnetic material or the magnetic particles are kneaded with the elastomer or the elastic material. Since during this step the relative content by percent (the percent by volume) of the powdery magnetic material or the magnetic particles relative to the binder component is difficult to increase, the coder must have a large thickness in order to secure the magnetic force sufficient to allow the magnetic sensor to perform sensing.
The molding of the coder made of the elastic material or the elastomer containing the powdery magnetic material or the magnetic particles, is performed by shaping with the use of a mold assembly appropriate to the shape of a product by means of, for example, an injection molding technique or a compressive molding technique and, in the case where a vulcanizing step is needed, the elastic material or the elastomer should be retained within the molding assembly for a vulcanizing time needed, thereby posing a problem in that a relatively large number of processing steps are needed.
Also, the coder made of the elastic material or the elastomer containing the powdery magnetic material or the magnetic particles, requires the detecting sensor to be positioned at a location in the vicinity of and relative to the slinger used therein in a direction axially of such slinger so that, for example, in the bearing seal device utilizing the rotation detecting device for detecting the rotational speed of the wheel, the space for mounting thereof can be reduced and the detecting performance can be drastically increased. In such case, when particulate matter such as sand particles are trapped in and bitten within a gap delimited between the bearing seal surface on a rotating side and a detecting sensor surface on a stationary side during run of a motor vehicle, it is often observed that the surface of the coder made of the elastic material or the elastomer will be damaged considerably due to, for example, frictional wear.
In the case of the coder made of the plastic material (plastomer) containing the powdery magnetic material or the magnetic particles, when an attempt is made to mold the coder by the use of the conventional injection molding technique, the compressive molding technique, the extrusion molding technique using the T-shaped die, the sheet molding technique such as the calendaring technique, or the insert molding technique as hereinbefore discussed, the use of a synthetic resin component that serves as a binder for retaining the powdery magnetic material or the magnetic particles is needed after all. However, even where the synthetic resin component is used as the binder, as is the case with the elastomer, the dispersing step has hitherto been required in which prior to the shaping to the shape of the coder, the powdery magnetic material or the magnetic particles are kneaded together with the plastomer or the elastic material. After all since, during this dispersing step, it is difficult to increase the relative percent content (the percent by volume) of the powdery magnetic material or the magnetic particles relative to the binder component, the thicknesswise dimension of the coder has to be increased in order to secure the magnetic force sufficient to allow the magnetic sensor to perform a stable sensing operation.
Also, when a preform material prepared by kneading a mixture of the powdery magnetic material or the magnetic particles and the plastomer or the elastic material according to the conventional manufacturing method is injected into or compressed within the mold assembly to form the coder, or is shaped by means of the insert molding technique to form the coder, since the magnetic particulate component contained in the preform material is an oxide of metal and is therefore so hard as to bring about a problem associated with frictional wear of molds of the mold assembly and/or the molding machine in term of mass productivity and since the preform material containing a high content of the magnetic particulate component tends to exhibit a high melt viscosity, there has been a problem in that the molding pressure or the mold clamping force has to be increased, resulting in increased load on the molding.
Even in the case of the extrude molding using the T-shaped die and the sheet molding such as the calendaring technique, since the magnetic particulate component contained in the preform material is an oxide of metal and is therefore hard, there has been a problem associated with frictional wear of the T-shaped die and rolls of the calendaring machine in terms of mass productivity.
In view of the foregoing, the present invention is intended to provide a magnetic encoder capable of being reduced in thickness, being excellent in resistance to frictional wear and, also, in productivity.
Another important object of the present invention is to provide a wheel bearing assembly, which is effective to achieve the rotation detection with a simplified structure with no need to increase the number of component parts used and, also, to employ the magnetic encoder for detection of the rotation that has an increased durability.
In order to accomplish these objects of the present invention, there is, in accordance with one aspect of the present invention, provided a magnetic encoder which includes a multi-pole magnet element having a plurality of opposite magnetic poles alternating with each other in a circumferential direction; and a core metal for supporting the multi-pole magnet element. The magnetic encoder of this structure is featured in that the multi-pole magnet element is a sintered element prepared by sintering a powdery mixture of a powdery magnetic material and a powdery non-magnetic metallic material. The multi-pole magnet element may be of, for example, of an annular shape such as a ring shape or of a disc shape. Similarly, the core metal is also of an annular shape such as a ring shape or of a disc shape.
According to this aspect of the present invention, since the multi-pole magnet element is a sintered element in which a powdery mixture of the powdery magnetic material and the powdery non-magnetic metallic material is sintered, the following advantages can be obtained.
a) As compared with the conventional elastomer and plastomer, it is possible to increase the ratio of the powdery magnetic material used and, for this reason, a magnetic force per unit volume can be increased. Thus, the detecting sensitivity can be increased and, also, the thicknesswise dimension can be reduced.
b) As compared with the conventional sintered magnet in which only the powdery magnetic material is sintered, cracking does hardly occur due to the presence of the powdery non-magnetic metallic material serving as a binder.
c) Since the surface is hard as compared with the conventional elastomer or the like, the resistance to frictional wear is excellent and damages are hard to occur.
d) As compared with the conventional elastomer or the like, the productivity is excellent.
In the magnetic encoder according to the present invention, the powdery magnetic material may be a ferrite powder. The ferrite powder is inexpensive as compared with the other powdery magnetic material and, accordingly, the use of it make it possible to manufacture the magnetic encoder inexpensively. The ferrite powder may be a mass of particulates or a pulverized powder of a wet-type anisotropic ferrite core. Where the pulverized powder of the wet-type anisotropic ferrite core is used, it is necessary to prepare a green compact formed from a powdery mixture with the powdery non-magnetic metallic material in a magnetic field. The green compact stands for an unsintered green material.
Also, he powdery magnetic material may be a magnetic powder of a rare earth type. By way of example, it may be a magnetic powder of a samarium type or a magnetic powder of a neodymium type. Where the magnetic powder of a samarium type or the magnetic powder of a neodymium type is employed, a high magnetic force can be obtained. The magnetic powder of the samarium type referred to above may be a magnetic powder of a samarium iron (SmFeN) type and the magnetic powder of the neodymium type referred to above may be a magnetic powder of a neodymium iron (NdFeB) type. The powdery magnetic material may also be suitably employed in the form of a gas atomized powder of manganese aluminum (MnAl).
The powdery non-magnetic material that can be employed in the practice of the present invention may be a powder of stainless steel or a powder of tin. Where the powdery magnetic material is employed in the form of a ferrite powder, either the stainless steel powder or the tin powder can be used for the powdery non-magnetic metallic material. On the other hand, where the powdery magnetic material is employed in the form of the magnetic powder of the samarium type, either the stainless steel powder or the tin powder can be also used for the powdery non-magnetic metallic material. Again, where the powdery magnetic material is employed in the form of the magnetic powder of the neodymium type, either the stainless steel powder or the tin powder can be similarly used for the powdery non-magnetic metallic material. As compared with the other non-magnetic metallic powder, the stainless steel powder is excellent in rust prevention and, therefore, the sintered element utilizing the stainless steel powder can exhibit a high rust preventive property.
The powdery mixture referred to above may contain two or more powdery magnetic materials or two or more powdery non-magnetic metallic materials. Also, the powdery mixture referred to above may contain two or more powdery magnetic materials in combination with two or more powdery non-magnetic metallic materials. Where the powdery mixture referred to above contains two or more powdery magnetic materials or two or more powdery non-magnetic metallic materials, a desired property can be obtained by mixing a plurality of arbitrarily chosen powders. By way of example, where the sole use of the ferrite powder appears to result in an insufficient magnetic force, the ferrite powder may be mixed with a required amount of the magnetic powder of the samarium type or the magnetic powder of the neodymium type that is a rare earth magnetic material so that, while being manufactured inexpensively, the magnetic force can be increased.
The magnetic powder containing two or more materials may be a mixture of two or more of the magnetic powder of the samarium iron (SmFeN) type, the magnetic powder of the neodymium iron (NdFeB) type, and the gas atomized powder of manganese aluminum (MnAl). By way of example, the powdery magnetic material referred to above may be any of a mixture of the magnetic powder of the samarium iron (SmFeN) type and the magnetic powder of the neodymium iron (NdFeB), a mixture of the magnetic powder of the neodymium iron type and the gas atomized powder of manganese aluminum, a mixture of the gas atomized powder of manganese aluminum and the magnetic powder of the samarium iron type, or a mixture of the magnetic powder of the samarium iron type, the magnetic powder of the neodymium iron type and the gas atomized powder of manganese aluminum. Also, the powdery magnetic material referred to above may be a mixture of the ferrite powder mixed with a required amount of any one of the magnetic powder of the samarium iron (SmFeN) type and the magnetic powder of the neodymium iron (NdFeB) type.
Preferably, the powdery magnetic material and the powdery non-magnetic metallic material both used in the powdery mixture have an average particle size not smaller than 10 xcexcm and not greater than 150 xcexcm. If one or the both of the powdery magnetic material and the powdery non-magnetic metallic material has or have an average particle size smaller than 10 xcexcm, the powdery mixture will hardly flow into the mold assembly when the green compact is to be prepared, and no green compact of a predetermined shape cannot be obtained. On the other hand, if one or the both of the powdery magnetic material and the powdery non-magnetic metallic material has or have an average particle size greater than 150 xcexcm, the green compact will not have a sufficient strength.
Also preferably, with respect to a composition of the powdery mixture, the volume based content of the powdery non-magnetic metallic material is not smaller than 1 vol. % and not greater than 90 vol. %. If the volume based content of the powdery non-magnetic metallic material is smaller than 1 vol. %, the amount of the powdery non-magnetic metallic material acting as a metal binder will be so insufficient that the resultant multi-pole magnet element obtained after sintering will become rigid, but fragile. It may occur that no green compact can be molded. On the other hand, if the volume based content of the powdery non-magnetic metallic material is greater than 90 vol. %, the amount of the powdery magnetic material will be relatively small and it will therefore be difficult to secure the magnetic force necessary to achieve a stabilized sensing.
Again preferably, the multi-pole magnet element made of the sintered element has a coefficient of linear expansion not lower than 0.5xc3x9710xe2x88x925 and not higher than 9.0xc3x9710xe2x88x925. If the multi-pole magnet element has a coefficient of linear expansion lower than 0.5xc3x9710xe2x88x925 or higher than 9.0xc3x9710xe2x88x925, the difference between it and a coefficient of linear expansion of a metallic material for the core metal is so large that the difference in amount of change in dimension when used under an environment of high or low temperature will become large. For this reason, there is a possibility that the multi-pole magnet element may be damaged, making it difficult to secure fixing between the multi-pole magnet element and the core metal.
The green compact of the powdery mixture before being sintered may have a porosity of not lower than 5 vol. % and not higher than 30 vol. %. If the porosity is lower than 5 vol. %, there is a high possibility that the green compact (the green compact) being prepared may break by the effect of a spring back phenomenon that will be induced as a result of an elastic restoration from an elastically deformed state of powdery mixture when the compacting or molding pressure applied is progressively removed. On the other hand, if the porosity thereof is higher than 30 vol. %, the physical strength of the sintered element will be so low that it is difficult to mechanically fix the sintered element to the core metal by means of a clamping process or a press fitting process and, also, because of lack of a sufficient bondability among the particles, there is a possibility that the green compact cannot be molded.
The sintered element forming the multi-pole magnetic element may preferably have a plate thickness not smaller than 0.3 mm and not greater than 5 mm. Considering that the powdery magnetic material and the powdery non-magnetic metallic material both used in the practice of the present invention are expensive, the smaller the plate thickness, the better. However, if the plate thickness is smaller than 0.3 mm, the green compact is difficult to mold. On the other hand, if the plate thickness is too large, variation in density will easily occur in the green compact to such an extent as to result in the green compact after having been sintered that is susceptible to deformation. In view of these considerations, the plate thickness is preferably within the range of 0.3 to 5 mm.
In the magnetic encoder of the structure discussed above with or without one or some of the various preferred features incorporated therein in accordance with the present invention, a surface of the multi-pole magnet element made of the sintered element may be formed with a rust preventive coating made of a high anti-corrosion clear paint. Preferably, the rust preventive coating may have a film thickness not smaller than 0.5 xcexcm and may also be formed by the use of a paint of a modified epoxy phenol hardening system as the high anti-corrosion clear paint.
Where the rust preventive coating is formed on the surface of the multi-pole magnet element, because of its rust preventive property, the magnetic encoder can be advantageously used in an environment where rusting tends to occur such as in a wheel bearing assembly. The paint referred to above can be expected to bring about an effect as a bonding agent for bonding the core metal and the sintered element together and, when penetrating into the pore in a surface region of the porous sintered element, the paint can be appropriately retained in the surface by an anchoring effect of the clear paint film component and, therefore, a favorable bondability as the rust preventive coating can be maintained even during the use for a prolonged period of time.
The powdery magnetic material and the powdery non-magnetic metallic material are mixed in a predetermined mixing ration by the use of a powder mixing machine to provide the powdery mixture which is subsequently compacted at normal temperatures within a mold assembly to thereby provide a green compact.
At this time, since the sintered element made of the powdery magnetic mixture containing the powdery magnetic material with the powdery non-magnetic metallic material used as a binder can provide a dry blend of the powders in which the powdery non-magnetic metallic material and the powdery magnetic material are dispersed in the powder mixing machine while the mixing ratio thereof is adjusted, the relative content (the volume based percent) of the powdery magnetic material in the sintered element can be increased. For this reason, the magnetic force effective to achieve the stabilized sensing can easily be obtained in the magnetic sensor and there is no need to increase the thickness of the multi-pole magnet element.
Moreover, even during the manufacture of the sintered element that subsequently forms the multi-pole magnet element, the sintering and molding method of the mixed powders based on the dry blend of the powders does not require any vulcanizing as compared with the injection molding or the compressive molding in the case of the conventional elastomer or the elastic material and involves a little load in molding and, therefore, the process of production can be extremely simplified. In addition, in the case of the molding of the green compact by means of the sintering process, as compared with the injection molding or the compressive molding of the elastomer or the elastic material, there is no problem associated with frictional wear of the mold assembly.
Yet, considering that the mounting of the sintered element forming the multi-pole magnet element onto the core metal can be achieved by the use of a mechanical fixing technique such as, for example, a simple crimping process or a interference fit, the reliability can be retained even when exposed to a severe conditions under a high and low temperature environment.
When the sintered element secured to the core metal as hereinbefore described is magnetized to have a plurality of opposite magnetic poles alternating with each other in a circumferential direction, the multi-pole magnet element results in.
The foregoing magnetic encoder, when positioned in face-to-face relation with the magnetic sensor, can be used for detecting rotation. When the magnetic encoder is rotated, passage of the opposite magnetic poles of the multi-pole magnet element can be detected by the magnetic sensor which detects the rotation in the form of a train of pulse, the number of which pulse corresponds to the number of revolutions and, hence, the rotational speed of the magnetic encoder. Since the multi-pole magnet element is made of the sintered element in which the powdery magnetic material has been mixed, it can be thin-walled while securing the magnetic force sufficient to obtain the stabilized sensing and, not only can the magnetic encoder be manufactured compact in size while having an excellent resistance to frictional wear, but also the multi-pole magnet element can be integrated together with the core metal, made of a metallic material, by the use of any known fixing method such as a crimping or press fitting, and thus it is excellent as a fixing method.
A wheel bearing assembly according to another aspect of the present invention makes use of the magnetic encoder with or without one or some of the various preferred features incorporated therein in accordance with the present invention. Accordingly, rotation detection can be achieved with a compact structure and the magnetic encoder for the rotation detection is robust.
As is well known to those skilled in the art, the wheel bearing assembly is very often exposed to a severe environment on a road surface where particles such as, for example, sand particles tend to be trapped in between the magnetic encoder and the magnetic sensor facing the magnetic encoder. Once this occurs, protection is available in the following manner.
Specifically, the surface hardness of the multi-pole magnet element made of the sintered element made up of the powdery magnetic material and the powdery non-magnetic metallic material is high as compared with the conventional coder made of the elastic material or the elastomer containing the powdery magnetic material and the magnetic particles. For this reason, in the wheel bearing assembly incorporating the magnetic encoder for the detection of the rotational speed of the wheel, even when the particles such as sand particles are trapped in and bitten within the gap between the surface of the multi-pole magnet element on the rotatable side and the surface of the magnetic sensor on the stationary side during run of the motor vehicle, there is a considerably high effect of reducing frictional damages to the multi-pole magnet element.
The wheel bearing assembly according to the present invention may employ the magnetic encoder as a part of a sealing unit for sealing a bearing space. By way of example, this wheel bearing assembly for rotatably supporting a wheel relative to a vehicle body may include an outer member having an inner peripheral surface formed with a plurality of rows of first raceways, an inner member having a corresponding number of second raceways defined therein in alignment with the first raceways in the outer member, and rows of rolling elements rollingly received in part within the first raceways and in part within the second raceways, wherein a sealing unit may be provided for sealing an annular space delimited between the outer member and the inner member. With this structure, the rotation of the wheel can be detected with no need to increase the number of component parts.
In such case, the sealing unit may include a first sealing plate mounted on one of the outer and inner members which serves as a rotatable member, and a second sealing plate of a generally L-sectioned configuration mounted on the other of the outer and inner members which serves as a stationary member, and positioned in face-to-face relation with the first sealing plate. The second sealing plate may have a side sealing lip fixed thereto and slidingly engaged with a radial upright wall of the first sealing plate and at least one radial sealing lip fixed to the second sealing plate and slidingly engaged with a first cylindrical wall of the first sealing plate, whereas the first sealing plate defines the core metal in the magnetic encoder with the multi-pole magnet element provided on the radial upright wall in overlapping relation at least in part therewith.
The first sealing plate referred to above is preferably of a generally inverted Z-shaped section having a first cylindrical wall on a mounting side that is mounted on the rotatable member, a radial upright wall and a second cylindrical wall. Also, the first sealing plate may be of a generally L-sectioned configuration.
Where the wheel bearing assembly is so constructed as hereinabove described, since one of the component parts of the sealing unit is defined by the magnetic encoder, the rotation of the wheel can be detected with a further compact structure with no need to increase the number of component parts. Also, where the magnetic encoder is incorporated in the sealing unit in the manner described above, although a problem may arise that in view of the magnetic encoder exposed to the above discussed road environment the sand particles may be trapped in between the magnetic encoder and the magnetic sensor, this problem can be substantially eliminated by the fact that since the surface hardness of the multi-pole magnet element is high as hereinbefore discussed, an effect of reducing possible frictional damages can be obtained. Yet, in the case of this structure described above, in view of the fact that the side and radial sealing lips fixed to the second sealing plate slidingly engage the first sealing plate, an excellent sealing effect can be obtained.
Where the first sealing plate is made of a generally inverted Z-shaped configuration as hereinbefore described, any one of the following structures can advantageously be employed.
For example, the radial upright wall of the first sealing plate may be of an axially stepped shape having an inner peripheral side portion and an outer peripheral side portion offset axially relative to each other.
The multi-pole magnet element may be fixedly crimped by the second cylindrical wall of the first sealing plate.
The second cylindrical wall of the first sealing plate may be formed with a plurality of plastically deformed portions that are plastically deformed at circumferentially spaced locations so as to protrude and, in such case, the multi-pole magnet element may be fixed to the first sealing plate by means of the plastically deformed portions. The plastically deformed portions may be formed by, for example, staking.
The second cylindrical wall of the first sealing plate may be formed with a plurality of tongues at circumferentially spaced locations so that the multi-pole magnet element can be fixed to the first sealing plate by means of plastic deformation of the tongues.
In the wheel bearing assembly according to the present invention, at least one of the multi-pole magnet element of the magnetic encoder and the core metal may preferably be treated with a rust preventive treatment. This rust preventive treatment can be performed by forming a rust preventive coating of a high anti-corrosion clear paint.